Electric field proximity detectors are well known. Various types of detectors have been documented which utilize electric fields to sense the presence of conductive or partially conductive objects entering their effective field of detection. Generally, objects are detected by measuring signal changes resulting from perturbations in a generated electric field. Electric field proximity detectors are advantageous in that they are generally safe, fast, inexpensive, and capable of measuring object distance to a high degree of resolution.
One specific variety of electric field sensing proximity detector is the so-called transmit-receive type. In a typical configuration, this variety of detector generates a low-frequency electric field at one transmitting electrode and detects the electric field at a second receiving electrode. Intruding objects affect the degree of capacitive coupling between the transmitting electrode and receiving electrode, and as a result their presence may be detected by measuring changes in the signal at the receiving electrode.
Known transmit-receive detectors operate predominantly in two modes. The first, or transmitter, mode is triggered when objects with high impedance to circuit ground (that is, electrically floating objects) enter the generated electric field. In this mode, the floating object behaves essentially as a conductive extension of the transmitting electrode. Electrical charge from the generated electric field travels along the object towards the receiving electrode more efficiently than it does through free space, and as a result, the capacitive coupling between the transmitting electrode and the receiving electrode increases. Accordingly, the signal at the receiving electrode also increases, and the proximity of the floating object is thereby indicated. (The foregoing assumes that the floating object is not so large as to behave like an earth-grounded object.)
The second, or shunt, mode is triggered when objects with low impedance to circuit ground (that is, objects grounded to circuit ground) enter the generated electric field. In this mode, the capacitive coupling between the transmitting electrode and the receiving electrode decreases as the grounded object draws near because the intruding object effectively shunts the intercepted electric field to circuit ground. Accordingly, the signal at the receiving electrode decreases as the object approaches, and the proximity of the grounded object is thereby indicated.
The existence of two separate modes of operation in known transmit-receive proximity detectors as described has two notable disadvantages. The first drawback is the added complexity in detector circuitry that is required to detect both floating and grounded objects. Because floating objects trigger an increase in current at the receiving electrode whereas grounded objects trigger a decrease in current, detector circuitry must be capable of detecting both of these types of signal changes. This requirement may add to the complexity of detector circuitry, especially if the device is intended to be capable of distinguishing between floating versus grounded objects.
The second and more serious drawback of the two operating modes of known detectors is the resulting inability of these devices to reliably sense objects with intermediate degrees of ground impedance. Again, because objects with high impedance to earth ground trigger an increase in current at the receiving electrode whereas objects with low impedance to circuit ground trigger a decrease in current, it follows that, for certain objects with intermediate impedance to circuit ground, no perceptible change in current will occur at the receiving electrode. Objects having such a degree of ground impedance may therefore, disadvantageously, escape detection. As a result, known transmit-receive proximity detectors are poorly suited for many safety applications.
Still another drawback of known transmit-receive type detectors is their susceptibility to stray capacitance. Component circuitry associated with the transmission and detection of the electric field in these devices may emit stray electric fields and capacitances that influence the measured signal at the receiving electrode. As a result, this stray capacitance may jeopardize proper object detection.
Hence what is needed is a transmit-receive type electric field proximity detector that is capable of reliably detecting objects, regardless of their impedance to circuit ground, that additionally has a reduced susceptibility to the corrupting effect of stray capacitance, and that optimally provides a single characteristic signal change for all detected objects.